In automobile engines, water cooling type cooling systems using a radiator are generally used in order to cool the engine. Conventionally, furthermore, in cooling systems of this type, a control valve, e.g., a thermostat, which controls the amount of cooling water that is circulated on the radiator side so that the temperature of the cooling water that is introduced into the engine can be controlled, has been used in order to improve the fuel consumption of the automobile. Thermostats that use a thermal expansion body as an actuator for controlling the valve, thermostats that depend on electronic control and the like are known as such thermostats.
Such thermostats are devices which can control the temperature of the cooling water to a specified state by interposing a valve part in a portion of the cooling water passage, closing this valve part and circulating the cooling water through a bypass passage without passing the cooling water through the radiator in cases where the cooling water temperature is low, and opening this valve part so that the cooling water is circulated through the radiator in cases where the cooling water temperature is high
It is generally known that an improvement in automobile fuel consumption can be achieved by lowering the cooling water temperature in cases where the automobile engine is operated under a high load, and raising the cooling water temperature in the case of a low load.
Under such conditions, electronically controlled valves, i.e., electronically controlled thermostats, have been used in recent years in order to provide the optimal water temperature for improving the fuel consumption of automobiles. Such electronically controlled thermostats are devices which control the cooling water temperature by arbitrarily controlling the degree of opening of the valve part, and controlling the cooling fan that is attached to the radiator, and which can thus perform appropriate control of the cooling water temperature. The reason for this is that the control device (engine control module) that variably controls the abovementioned electronically controlled thermostat can perform a control action while also obtaining various parameters in the engine control unit, e.g., detected information such as the cooling water temperature, outside air temperature, vehicle speed, engine rpm, degree of opening of the throttle and the like.
Various devices have been proposed in the past as devices that can achieve an improvement in fuel consumption by performing such cooling water temperature control under specified conditions.
For example, electronically controlled thermostats which are devised so that a heat-radiating element is mounted in the temperature sensing part of the thermostat, and quick heating of the cooling water during engine starting and an improvement in the fuel consumption of the engine are achieved by using heat radiation control of this heat-radiating element in combination, have already been proposed in the past (for example, see Japanese Patent Application Laid-Open No. 2001-317355).
As was described above, problems that must be considered when controlling the cooling water temperature in an electronically controlled thermostat include the “response characteristics from the setting of the powering of the actuator to the variation in the water temperature”.
Specifically, in conventional electronically controlled thermostat control methods, various factors such as undershooting, overshooting, hunting, rate of heat exchange of the cooling water and the like have on the time period extending from the powering of the actuator to the opening of the valve so that the actual water temperature shifts to the target water temperature; accordingly, a considerable time is required for this operation.
Conventionally, furthermore, the electrical circuit used to power the PTC used as a heat generating device mounted on the abovementioned actuator has been a constant-voltage circuit; accordingly, since the resistance value of the PTC varies with the temperature, a constant heat radiation quantity cannot be ensured. For example, assuming that the powering of the PTC is 10 W at 0° C., the powering is 5 W when the temperature of the PTC reaches 100° C.
Furthermore, in automobiles, the following problem also arises: namely, because of individual differences in the manner in which the driver rides, the layout of the cooling water circulation system in each vehicle, differences in thermostats and the like, it is difficult to determine the best set water temperature for the vehicle in the design stage; accordingly, it is desirable that attention also be given to such points.
The present invention was devised in light of such facts; it is an object of the present invention to obtain an electronically controlled thermostat control method which makes it possible to overcome the abovementioned problems encountered in the past, and to obtain an electronically controlled thermostat control method which makes it possible to realize high cooling water temperature tracking characteristics with a high degree of precision at a low cost.
Furthermore, it is another object of the present invention to obtain an electronically controlled thermostat control method which makes it possible to supply a set water temperature that is always suited to the vehicle, and to realize optimal fuel consumption and optimal powering, at a low cost, by providing engine load judgment means or a learning function.